[PATCH bpf-next v2 1/3] riscv, bpf: Fix support for BPF_SDIV and BPF_SMOD in RV32 JIT

[Pu Lehui]

lgtm, just some nits

On 2026/5/12 6:16, Kuan-Wei Chiu wrote:
> The current rv32 bpf jit compiler incorrectly treats BPF_SDIV and
> BPF_SMOD as unsigned operations. The BPF instruction set allows
> signed division and modulo by reusing the BPF_DIV and BPF_MOD opcodes
> with the instruction offset set to 1.
> 
> Update the emit_alu_r32() function to accept an 'is_sdiv' variable and
> emit the correct div and rem instructions when the offset is 1.
> 
> Before this patch:
> [   44.161771] test_bpf: #165 ALU_SDIV_X: -6 / 2 = -3 jited:1 ret 2147483645 != -3 (0x7ffffffd != 0xfffffffd)FAIL (1 times)
> [   44.167385] test_bpf: #166 ALU_SDIV_K: -6 / 2 = -3 jited:1 ret 2147483645 != -3 (0x7ffffffd != 0xfffffffd)FAIL (1 times)
> [   44.171053] test_bpf: #169 ALU_SMOD_X: -7 % 2 = -1 jited:1 ret 1 != -1 (0x1 != 0xffffffff)FAIL (1 times)
> [   44.172081] test_bpf: #170 ALU_SMOD_K: -7 % 2 = -1 jited:1 ret 1 != -1 (0x1 != 0xffffffff)FAIL (1 times)
> 
> After this patch:
> [   16.002192] test_bpf: #165 ALU_SDIV_X: -6 / 2 = -3 jited:1 95 PASS
> [   16.002983] test_bpf: #166 ALU_SDIV_K: -6 / 2 = -3 jited:1 1059 PASS
> [   16.017167] test_bpf: #169 ALU_SMOD_X: -7 % 2 = -1 jited:1 136 PASS
> [   16.023002] test_bpf: #170 ALU_SMOD_K: -7 % 2 = -1 jited:1 109 PASS
> 
> Fixes: ec0e2da95f72 ("bpf: Support new signed div/mod instructions.")
> Signed-off-by: Kuan-Wei Chiu <visitorckw at gmail.com>
> ---
>   arch/riscv/net/bpf_jit_comp32.c | 19 ++++++++++++++-----
>   1 file changed, 14 insertions(+), 5 deletions(-)
> 
> diff --git a/arch/riscv/net/bpf_jit_comp32.c b/arch/riscv/net/bpf_jit_comp32.c
> index 592dd86fbf81..7396899ea276 100644
> --- a/arch/riscv/net/bpf_jit_comp32.c
> +++ b/arch/riscv/net/bpf_jit_comp32.c
> @@ -509,7 +509,7 @@ static void emit_alu_r64(const s8 *dst, const s8 *src,
>   }
>   
>   static void emit_alu_r32(const s8 *dst, const s8 *src,
> -			 struct rv_jit_context *ctx, const u8 op)
> +			 struct rv_jit_context *ctx, const u8 op, bool is_sdiv)

let's combind `op` and `is_sdiv` to `const struct bpf_insn *insn`, and 
better to name `is_signed = insn->off == 1`

>   {
>   	const s8 *tmp1 = bpf2rv32[TMP_REG_1];
>   	const s8 *tmp2 = bpf2rv32[TMP_REG_2];
> @@ -539,10 +539,16 @@ static void emit_alu_r32(const s8 *dst, const s8 *src,
>   		emit(rv_mul(lo(rd), lo(rd), lo(rs)), ctx);
>   		break;
>   	case BPF_DIV:
> -		emit(rv_divu(lo(rd), lo(rd), lo(rs)), ctx);
> +		if (is_sdiv)
> +			emit(rv_div(lo(rd), lo(rd), lo(rs)), ctx);
> +		else
> +			emit(rv_divu(lo(rd), lo(rd), lo(rs)), ctx);
>   		break;
>   	case BPF_MOD:
> -		emit(rv_remu(lo(rd), lo(rd), lo(rs)), ctx);
> +		if (is_sdiv)
> +			emit(rv_rem(lo(rd), lo(rd), lo(rs)), ctx);
> +		else
> +			emit(rv_remu(lo(rd), lo(rd), lo(rs)), ctx);

how about
emit(is_signed ? rv_rem(lo(rd) : rv_remu(lo(rd), lo(rd), lo(rs)), ctx)

>   		break;
>   	case BPF_LSH:
>   		emit(rv_sll(lo(rd), lo(rd), lo(rs)), ctx);
> @@ -959,6 +965,7 @@ int bpf_jit_emit_insn(const struct bpf_insn *insn, struct rv_jit_context *ctx,
>   	u8 code = insn->code;
>   	s16 off = insn->off;
>   	s32 imm = insn->imm;
> +	bool is_sdiv = false;
>   
>   	const s8 *dst = bpf2rv32[insn->dst_reg];
>   	const s8 *src = bpf2rv32[insn->src_reg];
> @@ -1041,7 +1048,9 @@ int bpf_jit_emit_insn(const struct bpf_insn *insn, struct rv_jit_context *ctx,
>   			emit_imm32(tmp2, imm, ctx);
>   			src = tmp2;
>   		}
> -		emit_alu_r32(dst, src, ctx, BPF_OP(code));
> +		if ((BPF_OP(code) == BPF_DIV || BPF_OP(code) == BPF_MOD) && insn->off == 1)
> +			is_sdiv = true;
> +		emit_alu_r32(dst, src, ctx, BPF_OP(code), is_sdiv);
>   		break;
>   
>   	case BPF_ALU | BPF_MOV | BPF_K:
> @@ -1065,7 +1074,7 @@ int bpf_jit_emit_insn(const struct bpf_insn *insn, struct rv_jit_context *ctx,
>   		 * src is ignored---choose tmp2 as a dummy register since it
>   		 * is not on the stack.
>   		 */
> -		emit_alu_r32(dst, tmp2, ctx, BPF_OP(code));
> +		emit_alu_r32(dst, tmp2, ctx, BPF_OP(code), false);
>   		break;
>   
>   	case BPF_ALU | BPF_END | BPF_FROM_LE: